1. Background of the Invention
The present invention relates to a lithographic apparatus, an interferometer and a device manufacturing method.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In such a lithographic apparatus, an interferometer might be used for measuring displacements with a high accuracy. U.S. Pat. No. 4,784,490 discloses a typical high stability plane mirror interferometer.
Although such an interferometer provides a highly accurate measurement, measurement errors may occur due to unflatness of the measurement mirror. When the measurement mirror is displaced, e.g., in a direction perpendicular to the measurement beam touching the measurement mirror, the measurement beam may touch the mirror at a different area on the mirror. Unflatness of the mirror may thus result in an error in the distance measurement by the interferometer.